The mitochondrial gene T-urf13 is found in maize with Texas male-sterile cytoplasm (cms-T), and encodes a maize mitochondrial protein. The DNA encoding URF13 (T-urf13) is thought to have arisen by multiple derived from four different origins. See Dewey et al., Cell, 44, 439 (1986). In maize the URF13 protein causes susceptibility to toxins produced by two fungal pathogens, Bipolaris maydis race T (formerly Helminthosporium maydis race T) and Phyllosticta maydis. Diseases caused by these pathogens have curtailed large-scale use of cms-T maize for the production of hybrid seed. Treating isolated cms-T maize mitochondria with the host-specific fungal toxins (T toxins) produced by B. maydis race T (BmT toxin) or P. maydis (Pm toxin) causes mitochondrial swelling, leakage of small molecules and ions, inhibition of malate-stimulated respiration, and uncoupling of oxidative phosphorylation. BmT and Pm have been purified and characterized. See Kono et al., Tetrahedron Lett., 24. 3803 (1983); Kono et al., Bioorg. Chem., 10, 206 (1981); Kono et al., Tetrahedron Lett., 21, 1537 (1981).
The interaction of URF13 and T toxin results in pore formation in the inner mitochondrial membrane, which causes membrane permeability. See Levings, Science 250, 942 (1990). Similar events are observed in Escherichia coil (E. coli) expressing the cloned T-urf13 gene where, after exposure to T toxins, spheroplast swelling, inhibition of respiration, and ion leakage occur. R. Dewey, et al., Science 239, 293 (1988); C. Braun, et al., Proc. Natl. Acad. Sci. USA 86, 4435 (1989); C. Braun et al., Plant Cell, 2, 153 (1990). Methomyl (S-methyl-N-[(methylcarbamoyl)oxy]thioacetimidate), the active ingredient in the DuPont insecticide LANNATE (TM), mimics the effects of T toxins on isolated cms-T maize mitochondria or E. coli expressing URF13. Dewey et al, Science, 239, 293 (1988); Koeppe et al, Science, 201, 1227 (1978). Many mutations in T-urf13, including several at nucleotide positions encoding amino acid residue 39, render E. coli expressing the mutant URF13 insensitive to T toxins or methomyl. Braun et al, Proc. Natl. Acad. Sci. USA 86, 4435 (1989); Mark E. Williams and Gerty C. Ward, personal communication.
URF13 has also been expressed in two heterologous eukaryotic systems. The T-urf13 gene was expressed in Saccharomyces cerevisiae where the gene was modified to direct URF13 import into mitochondria (Glab et al., Mol. Gen. Genet., 223, 24 (1990); Huang et al., EMBO J., 9, 339 (1990). In these cases URF13 accorded T toxin and methomyl sensitivity to the mitochondria. In contrast, when the URF13 protein was not modified to direct import into the mitochondria, it did not confer toxin sensitivity to whole yeast cells. URF13 conferred T toxin sensitivity to Nicotiana tabacum, however, when it was expressed in the cytoplasm without a mitochondrial targeting sequence. Von Allmen, et al., Mol. Gen. Genet. 229, 405 (1991).